In existing optical projection devices, white light sources or RGB monochromatic light sources are generally adopted as projection light sources. In a case that a RGB monochromatic light source is adopted, there is a need to perform a light mixing operation on three monochromatic lights, i.e., red light, green light and blue light. However, currently, the red light, the green light and the blue light are generally mixed by wavelength combiner(s) for the existing projection device. The application of wavelength combiner(s) normally results in extremely high cost, extremely high assembly requirements, strict assembly tolerance requirements, and low yield, thereby failing to realize mass production. Thus, to provide a front-end system with excellent quality and moderate price for projection device is an urgent problem need to be solved by technicians in this filed.